The Alphavirus genus within the Togaviridae family contains about 30 mosquito-borne, enveloped, positive-stranded RNA viruses, one third of which cause significant diseases in human and animals worldwide. For example, neurotropic alphaviruses such as western, eastern, and Venezuelan equine encephalitis viruses (WEEV, EEEV, and VEEV, respectively) infect the central nervous system (CNS) and can lead to severe encephalitis in humans with fatality rates of up to 70%, and where survivors often bear long-term neurological sequelae (Deresiewicz et al., N Engl J Med 1997; 336:1867-74; Earnest et al., Neurology 1971; 21:969-74). Neurotropic alphaviruses are also important members of the growing list of emerging or resurging public health threats (Gubler, Arch Med Res 2002; 33:330-42) and are listed as CDC and NIAID category B bioterrorism agents, due in part to numerous characteristics that make them potential biological weapons: (i) high clinical morbidity and mortality; (ii) potential for aerosol transmission; (iii) lack of effective countermeasures for disease prevention or control; (iv) public anxiety elicited by CNS infections; (v) ease with which large volumes of infectious materials can be produced; and (vi) potential for malicious introduction of foreign genes designed to increase alphavirus virulence (Sidwell et al., Antiviral Res 2003; 57:101-11).
There are currently no licensed vaccines or antiviral drugs for alphaviruses. Formalin inactivated vaccines for WEEV or EEEV and a live attenuated VEEV vaccine (TC-83 strain) are available on an investigational drug basis, and are limited primarily to laboratory workers conducting research on these viruses (Sidwell et al., supra). However, these vaccines have poor immunogenicity, require annual boosters, and have a high frequency of adverse reactions. The development of alternative live attenuated, chimeric, and DNA-based alphavirus vaccines is being actively pursued, and several candidates have been tested in animal models (Barabe et al., Vaccine 2007; 25:6271-6; Wu et al., Vaccine 2007; 25:4368-75; Nagata et al., Vaccine 2005; 23:2280-3; Schoepp et al., Virology 2002; 302:299-309; Turell et al., Am J Trop Med Hyg 1999; 60:1041-4; Wang et al., Vaccine 2007; 25:7573-81; Fine et al., Vaccine 2008; 26:3497-506; Turell et al., Am J Trop Med Hyg 2008; 78:328-32). Nevertheless, the broad clinical application of these newer generation vaccines is likely years away. Furthermore, combined active vaccination and antiviral therapy may be a more effective response to an outbreak due to either natural transmission or intentional exposure to a viral pathogen (Bronze et al., Curr Opin Investig Drugs 2003; 4:172-8).
Several compounds have been reported to inhibit alphavirus replication, including the nucleoside analogs ribavirin and mycophenolic acid (Malinoski et al., Virology 1981; 110:281-9), (−)-carbodine, triaryl pyrazoline (Puig-Basagoiti et al., Antimicrob Agents Chemother 2006; 50:1320-9), and seco-pregnane steroids from the Chinese herbs Strobilanthes cusia and Cynanchum paniculatum (Li et al., Proc Natl Acad Sci USA 2007; 104:8083-8).
Nevertheless, there is still a pressing need to identify new antiviral drugs to treat these virulent pathogens.